If all we were looking for were sharpness, then it would be cheaper to just buy a ton of razor blades. Unfortunately, there's more to it than that when dealing with food preparation - weight (heft) and shape make a huge difference (which leads to buying different knives for different purposes - you don't want to use a cleaver when a boning knife would be easier and produce cleaner results). For almost all food preparation activities, a razor blade is the wrong shape (flat edge) and does not have enough heft to actually cut through. The only application I would recommend using a razor blade for is slashing bread - but your paring knife (if kept sharp) can serve that purpose without the chance of contaminating your food with industrial grease (used in the packaging of bulk razor blades to keep them from rusting in storage).